The Sidney Kimmel Comprehensive Cancer Center (SKCCC) Mass Spectrometry Core uses mass spectrometry coupled to one (ID) and two (2D) dimensional separations by column chromatography or gel electrophoresis to identify, quantify or characterize proteins and their post-translational modifications, that are expressed in well characterized protein fractions from cancerous cells, tissues or body fluids. Techniques such as difference gel electrophoresis (DIGE), isobaric tag for relative and absolute quantitation (iTRAQ), tandem mass tags (TMT) and stable isotope labeling of amino acids in cell culture (SILAC) as well as non-labeling methods (MudPIT, multi-dimensional protein identification technology) are available for quantifying relative differences in protein expression and post-translational modifications, such as acetylation, glycosylation, phosphorylation, nitrosation, ubiquitination and novel cleavage sites. Also available are Core developed methods to characterize post-translational modifications, such as LCMS methods to accurately determine the mass of intact proteins, a selective fluorescent labeling of cysteines to detect oxidized or nitrosated cysteines, or enrichment strategies to map phosphorylation sites. This Core is a part of larger institutional resource and provides a Proteomics Specialist to assist Center investigators and their staff in preparing and analyzing their samples or in using the Core's multi-user equipment. Through the Proteomics Specialist, the Center investigators have access to all the staff and expertise in the Johns Hopkins School of Medicine Mass Spectrometry and Proteomics Facility. Lay: The Mass Spectrometry Core not only identifies and quantifies proteins of interest to center investigators, but also identifies and quantifies the protein modifications and the proteins with which they interact and by which they are regulated.